Offline Access to Analytics Reporting API

my Google app gets the offline access this code of javascript library.

auth2.grantOffline().then(function(resp) {
     // where to use this auth_code? expiry of it?
    let auth_code = resp.code;
});

But I don't see any usage of this auth_code, is there any example/sample code to use it? what is the expiry date of this offline token code? Can we use this auth_code with PHP libarary?

https://developers.google.com/analytics/devguides/config/mgmt/v3/quickstart/service-php

https://developers.google.com/identity/sign-in/web/reference#googleauthgrantofflineaccessoptions

Thanks

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